1. Field of the Invention
The present invention relates to hydraulic system for equipment, such as off-road construction and agricultural vehicles, and more particularly to apparatus controlling a variable displacement pump used in such systems in a manner that enables a selected hydraulic function to have priority with respect to using pressurized fluid provided by that pump.
2. Description of the Related Art
With reference to FIG. 1, a backhoe-loader 10 is a common type of earth moving equipment that has backhoe assembly 20 attached to the rear of a tractor 15. The backhoe assembly 20 comprises a bucket 12 attached to the end of an arm 13 which in turn is coupled by a boom 14 to the frame of a tractor 15. The bucket 12 can be replaced with other work heads. A first hydraulic actuator 16 causes the bucket 12 to tilt with respect to an arm 13, and a second hydraulic actuator 17 causes the arm to pivot at the remote end of the boom. The boom 14 is raised and lowered with respect to the frame of a tractor 15 by a third hydraulic actuator 18. A joint 21 enables the backhoe assembly 20 to pivot left and right with respect to the rear end of the tractor 15, which motion is referred to as “swing” or “slew”. A fourth hydraulic actuator 19 is attached on one side of the frame of the tractor 15 and to the boom 14 and provides the drive force for the pivoting motion of the backhoe assembly 20.
In the exemplary backhoe-loader 10, the first through fourth hydraulic actuators 16-19 are cylinder-piston assemblies, however other types of hydraulic actuators, such as a hydraulic motor can be used with the present invention. Also on larger backhoes, a pair of hydraulic cylinders are attached on opposite sides of the tractor 15 to pivot the backhoe assembly.
A pair of stabilizers 22, only one of which is visible in the drawing, are located on opposite sides of the rear of the tractor 15 and are lowered to the ground during digging to support the tractor. Additional hydraulic actuators 23 are employed to raise and lower the stabilizers 22. The front wheels 24 of the backhoe are steered by another hydraulic actuator, not visible in FIG. 1.
The backhoe-loader 10 also has a loader assembly 25 attached to the front of the tractor 15. The loader assembly 25 comprises a load bucket 27 pivotally coupled to the forward end of a lift arm 26 that has a rearward end that is pivotally coupled to the tractor 15. A lift hydraulic actuator 28 raises and lowers the lift arm 26 and a load hydraulic actuator 29 pivots the load bucket 27 up and down at the end of the lift arm 26.
The flow of hydraulic fluid to and from each of the hydraulic actuators 16-19, 23, 28 and 29 is supplied through valves that are controlled by the backhoe operator. The pressurized fluid to drive the hydraulic actuators is supplied by a pump that is driven by the engine of the tractor. For greater efficiency, a variable displacement pump is used so the pressure of the fluid can be varied to be no greater than the pressure level required to drive the hydraulic actuator against the load forces applied to them. At times when the cylinders are not operating or when only low pressure is required, the displacement of the pump can be set so that high pressure fluid will not be produced and then wasted by merely being dumped into the fluid tank of the hydraulic system. In order to achieve optimal efficiency, the displacement of the pump has to be controlled in relation to the level of pressure required to drive the hydraulic actuators.